1. Field of the Invention
This invention generally relates to flexible substrate based ball grid array (BGA) packages, and more particularly to flexible film substrates for use in forming flexible substrate based BGA packages and manufacturing methods thereof.
2. Description of the Related Art
FIG. 1 depicts a conventional flexible substrate based BGA package 100 typically utilizing a flexible film substrate 110 (see FIG. 2) for carrying a semiconductor chip 120. The flexible film substrate 110 is provided with a plurality of chip connection pads 110a arranged about the periphery of the semiconductor chip 120. The semiconductor chip 120 is securely attached onto the flexible film substrate 110 through a nonconductive adhesive (e.g., epoxy resin) 122 and electrically connected to the chip connection pads 110a through a plurality of bonding wires 130. The chip connection pads 110a are electrically connected to a plurality of solder pads 110b through conductive traces 110d. The flexible film substrate 110 has a plurality of through-hole 110c disposed corresponding to solder pads 110b. Each solder pad 110b has a portion exposed within the corresponding through-hole 110c for mounting a solder ball 140. The upper surface of the flexible film substrate 110, the semiconductor chip 120 and the bonding wires 130 are encapsulated in a package body 150. The package body 150 is formed from insulating material such as epoxy. The flexible substrate based BGA package 100 is mounted to a substrate (not shown), such as a printed circuit board, through the solder balls 140.
Typically, the surfaces of the chip connection pads 110a and solder pads 110b are plated with a layer of metal (not shown) such as gold which bonds well with conventional bonding wire material. However, gold does not bond well with the nonconductive adhesive 122, so the central part of the chip attaching area is not provided with solder pads in order to enhance the adhesion between the flexible film substrate 110 and the nonconductive adhesive 122, whereby the semiconductor chip 120 is more securely attached onto the flexible film substrate 110. However, since the solder pads 110b are not evenly distributed on the chip attaching area 160 of the flexible film substrate 110, the central part thereof is prone to be deformed by the stress due to CTE (coefficient of thermal expansion) mismatch between the central part and other part of the chip attaching area 160. This may create problems of delamination between the chip and the substrate or die cracking.